Amplitude modulation limiter circuit



March 21, 1944. V SHUNT AMPLITUDE MODULATION LIMITER cmcun' 'Filed Aug. 22, 1941 INVENTOR BY ww/l/ ATTORNEY Patented Mar. 21, 1944 AMPLITUDE MODULATION LIMITED omcm'r Seymour Hunt, Flushing, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application August 22, 1941, Serial No. 407,889

' 14 Claims.

My present'invention relates to limiter circuits, and more particularly to an amplitude limiter circuit adapted for use in receivers of freuency, or phase, modulated carrier waves.

One of the main objects of my present'invention is to provide a multi-grid tube which is arranged in a limiter circuit, used in a receiver of frequency, or phase, modulated carrier waves; the modulated carrier waves being impressed upon at least one of the control grid circuits of the tube including a time constant network which has a value which is short compared to the shortest audio period and is long compared to the wave period whereby amplitude modulation effects existing on the modulated carrier wave are minimized, a second one of the control grids of thetubes being connected to the time constant network so as to exert a further limiting action on the output amplitude of the which may be one of the' 6A8 type. The tube is provided with a cathode 2 and a first control grid 3, a second control grid 4 and a plate, or output, electrode't. The control grid 4 is located between a pair of positively biased screen grid electrodes, the lead to the screen grids being lay-passed to ground for carrier frequencies. ,The

u tube element between grid 3 and the screen grid to utilize in place of cascaded amplitude modu= lation limiters known in the prior art, a single tube provided with at least two spaced control grids, at least one of thecontrol grids having frequency, 6r phase, modulated carrier waves applied thereto, and both grids having applied to them fast-acting control voltage for eliminating amplitude modulation efiects which may exist on the modulated carrier wave.

Still other objects of my invention are to improve limiter circuits for frequency modulated carrier wave receivers, and more particularly to provide a multi-electrode limiter tube which is not only reliable and efficient in operation, but, also, is economically and effectively assembled-1n frequency modulation receivers.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically several circuit organis an unused electrode of the tube, The numeral 5 denotes the tuned input circuit which is connected between. the control grid 3 and the grounded cathode 2. In the low potentialside of the input circuit there is included a series resistor l which is shunted by condenser 8. The plate 5 is connected to the positive terminal of the direct current source through the output transformer primary winding 9 which i shunted by condenser 10. The primary circuit 9-") is tuned to the same center frequency of the incoming modulated carrier waves as is input circuit 6. The grid 4 is connected to an adjustable tap i I, which is slidable along resistor 1,

through a resistor 82 which is shunted by con-' denser IS. The network 12-!3 has the same time constant value as the network 1-8.

The input circuit 6 maybe coupled to a preceding resonant circuit 8'. The latter may be located in the plate circuit of the final intermediate frequency (I. F.) amplifier tube, or it may be located in the plate circuit of the converter tube. This will be feasible where the receiver is of the superheterodyne type, and, of course, it will be understood by those skilled in the art that the latter type of receiver is substantially universally employed in receiving frequency modulated carrier waves. 0! course, the frequency modulated (FM) carrier waves may be applied directly to an amplifier tuned to the center frequency of the carrier waves, and in such case amplified waves will be applied to the input circuit 6.

The presently assigned FM 'band covers a range of 42 to 50 megacycles (mo) and the permissible FM channel width is 200 kilocycles (km). of circuits 6'-6 and 9-H! would be tuned to the center, or mean, frequency of the FM waves. Where the stage preceding the circuit 6' is the I. F. amplifier, then the aforesaid tuned circuits will be tuned to the operating 1. F. value which a may be a frequency chosen from a range of 2 prior resonant circuits on the transmitted FM Each wave energy. Regardless of the cause, it is necessary to insure the impression upon the FM detector of modulated carrier waves which .have

no amplitude modulation components. Since the function of the FM detector is to produce modulation signal voltages corresponding to the modulation signal'voltage applied to the carrier wave at the transmitter, it is essential that at the detector input circuit there be no Variation in the amplitude of the carrier. As is well known the modulation signals appear on the FM carrier wave as a frequency deviation of the transmitter carrier frequency. In the past it has been proposed to use cascaded limiter circuits. Such proposals require extra circuit elements. By providing a dual control over the electron stream within a single tube, it is possible to use a single tube and secure more eflicient limiting action which can function at relatively low carrier input levels. This is a further advantage over prior' limiters, in that considerable carrier amplification has been thought necessary prior to impression of the. waves upon th limiter. net* work.

Considering the mode of operation of the limiter tube, it is pointed out that the cathode,

latter case there is provided the opportunity of preventing an otherwise drooping characteristic which is often encountered in the usual type of limiter circuit using but one control grid. Thus, in Fig. 2 there is shown a modification of the arrangement of Fig. 1, wherein the control grid 3 is connected to the tuned input circuit 6 in the same manner as Fig. 1; However, control grid 4 is connected to an adjustable tap ll through a direct current blocking condenser 30. The tap ll'may be adjusted to any desired point along the coil of input circuit 6 so that the relative magnitudes of the FM waves applied to grids 4 and 3 may be easily adjusted.

control grid 3 and plate 5 act like a low mu section, whereas the cathode, control grid 4 and platei. act in the manner of a high mu section. Accordingly, the limiter voltage developed across resistor! is applied at a predetermined magni tude to the control grid 4 so as to produce a limiter characteristic which is substantially flat above a relatively small carrier input level. It is pointed out that there is developed across resistor I, by virtue of the time constant network 8, a fast-acting automatic volume control (AVC) voltage. The network l2|3 is chosen so as to have atime constant which is short compared to the shortest audio period and long compared to the carrier wave period. Hence, the control grid 4 has applied to it fast-acting AVC bias which provides a final flattening control over the limiter characteristic should there otherwise be a tendency for the characteristic to rise. .01? course, the tap ll, may be adjusted to any desired portion of resistor 1 so that a predetermined value of limiter voltage may be applied to control grid 4. Itwill be noted, furthermore, that the FM waves are applied solely to control grid 3, while both grids 3 and 4 have the fast-acting limiting voltage applied thereto.

The limited FM waves developed in the output circuit Ill-9 are impressed upon the input circuit 20 of the FM detector. It is not believed necessary'to show the nature of the detector because those skilled in the art are fully aware of the various types of devices that may be employed to derive the modulation voltage from the limited FM waves. Besides providing the limiting operation at low carrier levels, this circuit functions to initiate limiting action relatively quickly and at a rate which is faster than would be the case applied to only oneof the control grids. In this The grid 4 is further connected to the adjustable tap ll through a series path including the carrier frequency choke coil 49 and the biasing voltage. source 4|. The voltage source 4| is poled so as to render grid 4 normally negative with respect to the cathode. By adjusting the tap II to any desired point along resistor 1 the magnitude of the limiter voltage applied to grid 4 will be controlled. It will, therefore, be seen that in the arrangement in Fig. 2 there are provided controls H and H which may be adjusted to determine the magnitude of the FM waves and the magnitude of the limiter voltage which is to be applied to grid 4. In this way the resultant limiter characteristic may be varied over a considerable range so as to vary in different manners with the variation in carrier amplitude level. The expression angle modulated when used in the claims in connection with the nature of the modulation of carrier .wave energy generically signifies frequency modulation, phase modulation or hybrids thereof.

While I have indicated and described several systems for carryingmy invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is! 1. In combination, in an amplitude modulation limiter stage, a tube provided with at least a cathode, an output electrode and a pair of intermediate control grids, an output circuit connected to the output electrode, a source of angle modulated carrier waves connected between at least one of the intermediate grids and the cathode, means connecting the second intermediate grid to a point on an input circuit including said first intermediate grid and cathode, and a separate time constant network being connected in circuit with each of said intermediate grids, each of said time constant networks having a value short compared 'to the highest modulation frequency and long compared to the carrier wave.

2. In combination, an electron discharge tun. provided with a cathode, a plate, and at least two control grids, an input circuit tuned to the center frequency of applied frequency modulated carrier waves, means connecting the cathode and one of the control grids to said input circuit, said means including a fast-acting time constant network developing a negative direct current voltage from said waves which is applied to said one grid for limiting the tube output, means providing an adjustable directcurrent voltage connection between the second control grid and a desired poin on said time constant network for applying a predetermined portion of said same negative voltage to the second grid thereby to augment said limit ing action.

3. In combination, an electron discharge tube provided with a cathode, a plate, and at least two control grids, an input circuit tuned to the center frequency of the frequency modulated carrier waves, means connecting the cathode and one of the control grids to said input circuit, said means including a fast-acting time constant network, means providing an adjustable direct current voltage connection between the second control grid and a desired point on said time constant network, and additional means providing an adjustable modulated carrier wave connection between the said input circuit and said second control grid.

4. In combination with a source of angle modulated carrier waves, a limiter stage for minimizing amplitude modulation effects in the carrier waves, said limiter including a multigrid electron discharge tube, a resonant circuit coupling said source to the cathode and one of the grids of said tube, said resonant circuit including a resistance-condenser network having a relatively short time constant value for developing a negative direct current limiting voltage, and a direct current voltage connection between a point on said time constant network and a second grid of said tube whereby the same limiting voltage developed across said time constant network is applied to said second grid thereby to augment said minimizing action.

5. In combination with a source of angle modulated carrier waves, a limiter stage for minimizing amplitude modulation effects in the carrier waves, said limiter including a multigrid electron discharge tube, a resonant circuit coupling said source to the cathode and one of the grids of said tube, said resonant circuit including a resistance-condenser network having a relatively short time constant value, and a direct current voltage connection between a' point on said time constant network and a second grid of said tube whereby limiter voltage developed across said time constant network is applied to said second grid, said direct current voltage connection including a time constant network having a relatively short time constant value.

6. In combinationya tube provided with at least a cathode, an output'electrode and a pair of intermediate control grids, an output circuit connected to the output electrode, a source of angle modulated carrier waves connected to at least one of the intermediate grids, means conprovided with 'a cathode, a plate, and at least two control grids, means connecting the cathode modulated carrier wave connection between the said input circuit and said second control grid.

9. In combination with a source of angle modulated carrier waves, a limiter including a multiwork connecting the cathode and one of the contion betwen the second control grid and a desired point on said time constant network for grid electron discharge tube, a circuit tuned to on said time constant network and a second grid of said tube whereby the same limiting voltage developed across said time constant network is applied to said second grid thereby to augment limiting action.

' 10. In combination, in an amplitude modulation limiter stage, a tube provided with at least a cathode, an output electrode and a pair of intermediate control grids arranged in the electron stream from the cathode to output electrode, an output circuit connected to the output electrode, a source of angle modulated carrier waves connected between one of the intermediate grids and the cathode, means connecting the second intermediate grid to a point in an input circuit including said first intermediate grid and cathode, separate time constant networks being connected in circuit with each of said intermediate grids, each of said time constant networks having a value short compared to the shortest modulation frequency and long compared to the carrier wave.

11. In combination, an electron discharge tube provided with a cathode, a plate, and at least two control grids, an input circuit tuned to the center frequency of the frequency modulated carrier waves, means connecting the cathode and one of the control grids to said input circuit, said means comprising resistance and shunt capacity acting as a fast acting time constant neti work for developing a negative direct current voltage, means providing an adjustable direct current voltage connection between the second control grid and a desired point on said time constant network for applying a predetermined portion of the same negative voltage to the second grid. J

12. In combination, an electron discharge tube provided with a cathode, aplate, and. at least two control grids, an input circuit tuned to the center frequency of frequency modulated carrier waves, means connecting the cathode and one of the control grids to said input circuit, said means consisting of resistance and capacity acting as a fast-acting time constant network, means providing an adjustable direct current voltage connection between the second control grid and a lated carrier wave connection between the said applying. a predetermined fraction of the same I negative voltage to said second grid.

8. In combination, an electron discharge tube input circuit and saidsecond control grid.

13. In combination with a source of angle modulated carrier waves, a limiter stage for mini-. mizing amplitude modulation effects in the car'- v and a direct current voltage connection between a, point on said time constant network and a second grid of said tube whereby limiting voltage developed across said time constant network is applied to said second grid.

14. In combination with a source of angle modulated carrier waves, a limiter stage for minimizing amplitude modulation efiects in the carrier waves, said limiter including a multi-grid electron discharge tube, a resonant circuit tuned to the modulated wave center frequency coupling said source to the cathode and one of the grids of said tube, said resonant circuit consisting of a resistor and shunt condenser network having a relatively short time constant value, a direct current voltage connection between a point on said time constant network and a second grid of said tube whereby limiter voltage developed across said time constant network is applied to said second grid, said direct current voltage connectlon including a second time constant network having a relatively short time constant value.

SEYMOUR HUNT. 

